1) Field on the Invention
The present invention relates to a process for producing benzoyl chlorides useful as raw materials and intermediate products of agricultural chemicals, chemical synthetic products, etc.
2) Prior Art
Hitherto, as processes for producing benzoyl chlorides, a process for conducting hydrolysis of the corresponding trichloride and a process for chlorinating the corresponding aromatic carboxylic acid as raw material with a chlorinating agent such as thionyl chloride, phosphorus pentachloride, etc., have been known. U.S. Pat. No. 3,894,923 discloses a process for producing benzoyl chloride by reacting benzaldehyde having no side chain with chlorine.
Among them, when alkyl benzoyl chlorides are produced according to the process for conducting hydrolysis of the corresponding benzo trichlorides, it is very difficult to conduct selectively such reaction since alkyl benzo trichloride as the raw material is necessary and only one methyl group on alkyl benzene having at least two alkyl groups need be chlorinated selectively in order to obtain alkyl benzo trichloride.
Further, the process for chlorinating alkyl benzene carboxylic acid with thionyl chloride, etc., provides a high yield of acid chloride, but the use of the chlorinating agent requires a higher cost than the case of using chlorine as the raw material and causes many problems in respect of refining and post treatment.
U.S. Pat. No. 5,599,981 describes a process for producing alkyl benzoyl chloride in a high yield by reacting alkyl benzaldehyde with chlorine in a liquid phase in a comparatively low temperature. However, in the process for reacting an aromatic aldehyde with chlorine in a liquid phase, benzal chlorides are by-produced depending upon reaction conditions. For example, in U.S. Pat. No. 3,894,923 which discloses to obtain benzoyl chloride by reacting benzaldehyde with chlorine, about 0.3% of benzal chloride to benzoyl chloride has been by-produced.
It is difficult to separate the intended benzoyl chlorides from benzal chlorides in a conventional operation such as distillation since the boiling point of benzal chlorides is close to that of benzoyl chlorides. For example, the specific volatility of ethyl benzoyl chloride to 1-dichloromethyl-4-ethyl benzene as benzal chlorides is 1.1 or below.
Therefore, the separation between both in a conventional industrial distillation is very difficult.
It is not preferable that a large amount of such impurity is mixed in the intended benzoyl chlorides since benzoyl chlorides are utilized as raw materials of agricultural chemicals, etc.
Further, the longer the reaction time or the higher the concentration of aromatic aldehyde as raw material in the reaction liquid is, the more the amount of benzal chlorides to be by-produced is increased.
In the process for reacting an aromatic aldehyde with chlorine in a liquid phase, exothermic reaction occurs. Therefore, when benzoyl chlorides are produced with an industrial scale according to said process, the reaction time cannot be shortened to an extreme from the problem of removal of heat or the problem of operation. It is not preferable in an industrial production to lower the concentration of aromatic aldehyde as the raw material in the reaction liquid since production efficiency is lowered.
Thus, even in case where the reaction time is industrially suitable and the concentration of aromatic aldehyde as the raw material is high, a process for producing benzoyl chlorides to suppress by-production of benzal chlorides has been required.